Atomic hydrogen (H•) is the smallest radical in the universe, consisting of one electron and one proton. Irving Langmuir observed H• experimentally for the first time in 1912, and it has since found applications in a variety of fields including solid state materials, quantum science, and cancer therapy.
But the extreme conditions historically used to generate atomic hydrogen—gamma rays, high-energy ultraviolet light, liquid mercury, temperatures hot enough to melt iron—have kept it largely out of the hands of synthetic chemists, even as organic radical reactions have enjoyed a resurgence.
“Extreme UV with mercury is not something you like to work with as a chemist at a pharma company,” says Roopender Kumar, an organic chemist at University College London.
Unwilling to let physicists have all the fun with atomic hydrogen, Kumar devised a way to generate H• under conditions accessible by the average synthetic lab (J. Am. Chem. Soc. 2026, DOI: 10.1021/jacs.6c07870). Thus freed, the radical can hydrogenate organic molecules without the need for H2 gas or expensive catalysts.
“We can do hydrogenation now without molecular hydrogen, without any palladium or any metal,” Kumar says, “enabled by the hydrogen atom, this tiny, beautiful element.”
To get to H•, Kumar took inspiration from another unusual species, known as a Rydberg radical. Rydberg radicals, which consist of a cationic core and a high-energy valence electron, are also used in quantum computing and have not been explored much for synthesis.
Kumar’s approach involves protonating the nitrogen-based compound hydrazine and then jamming in an extra electron using a simple thiol photocatalyst and ultraviolet light. The resulting species—which is proposed to be a Rydberg radical, but Kumar says more work would be needed to confirm—spits out H• when it decomposes. The H• then happily proceeds to hydrogenate any alkenes and halides in its vicinity.
Roopender Kumar harnessed unusual radical intermediates to accomplish hydrogenation reactions, like the one shown here, under mild, metal-free conditions.
Kumar and his collaborators at Imperial College London and the Max Planck Institute of Colloids and Interfaces used deuterium labeling to confirm that the new hydrogens on the organic products had come from the hydrazine. They also studied the mechanism using radical trapping and electron paramagnetic resonance spectroscopy.
Igor Alabugin, an expert in organic radicals at Florida State University, says the hydrogenation reaction is both creative and practical, and the researchers used “an impressive combination of tools” to convincingly show that H• is integral to the mechanism.
“I like it because it’s showing the practical importance of a class of molecules which really defy many of the conventional rules of valence and which expand the possibilities for chemical bonding,” says physical chemist Joseph Vincent Ortiz, an emeritus professor at Auburn University who spent his career studying unusual molecular species such as Rydberg radicals.
Kumar plans to keep refining the hydrogenation reaction conditions to be even more user friendly, “and of course the greatest thing would be if we can do it asymmetric,” he says. He also hopes to find more uses for Rydberg radicals in synthesis.
